In the installation of wall and floor covering surfaces such as ceramic tile, slate, marble and the like, numerous compositions have been developed which utilize a hydraulic cement such as Portland cement as the principal factor in the bonding adhesives. Ordinary Portland cement per se, or together with sand or limestone fillers, is generally ineffective for this purpose since it does not have the ability to retain the water used in mixing it for sufficient time to enable an adequate bonding cure to occur. In order to overcome this disadvantage, there was developed a type of adhesive now known in the art as dry-set Portland cement mortar. These mortars possess certain advantageous properties; they may be exposed to air after mixing with water for a period of time, they may be applied to the substrates without undue hurry, they remain plastic for a sufficient period of time to permit small movements to be made in the tile installation after the initial substrate to surface contact, they develop strong bonding between the tile or similar surfacing and the substrate surface to which it is bonded, and they possess a reasonably predictable initial set time, minimum drying shrinkage, and some sag resistance. A number of such dry-set compositions have been developed starting with the initial composition of one of the co-inventors herein as disclosed in U.S. Pat. No. 2,820,713.
One of the many properties required by mortar for setting tile is that it be sag resistant. Sag resistance is a term used in the trade and defined by a test method that is part of the American National Standard Specification for Dryset Portland Mortar Cement--A 118.1. Sag resistance is a property or characteristic relating to the ability of the mortar to resist movement under load until a certain load level is reached. This property or characteristic is vitally important in dry-set mortars and also to a latex Portland cement mortar since a mortar to be practically functional must be in a slurry or paste form on the one hand, but also must be capable of supporting the load imposed on it by the tiles being set. It is vital that the mortar support the tile without any appreciable sinking of the tile into the mortar during the period in which the mortar sets. In wall applications sag resistance is even more critical because the mortar must hold the tile in position on the wall during the period in which the mortar is setting.
In the course of development of dry-set mortars, it was found that the sag resistance, that is to say, the resistance to downward movement of an initially set tile during the curing process, could be substantially minimized by the addition to the composition of asbestos fibers. Since the time of this initial discovery, it has been found that asbestos possesses a long-term but hidden carcinogenic effect and therefore the use of asbestos, particularly in fiber form, has been strictly controlled if not prohibited under certain circumstances. It has therefore become most important in this particular art to find substitutes for asbestos fiber to provide the desired sag resistance. This problem has been solved in the present invention by the use of certain salts which under appropriate conditions form gelatinous or hydrated hydroxides.